Wideband Code Division Multiple Access (WCDMA) telecommunication systems have many attractive properties that can be used for future development of telecommunication services. A specific technical challenge in e.g. WCDMA and similar systems is the scheduling of enhanced uplink channels to time intervals where the interference conditions are favourable, and where there exist a sufficient capacity in the uplink of the cell in question to support enhanced uplink traffic. It is well known that existing users of the cell all contribute to the interference level in the uplink of WCDMA systems. Further, terminals in neighbour cells also contribute to the same interference level. This is because all users and common channels of a cell transmit in the same frequency band when Code Division Multiple Access (CDMA) technology is used. The load of the cell is directly related to the interference level of the same cell.
In order to retain stability of a cell, and to increase the capacity, fast enhanced uplink scheduling algorithms operate to maintain the load below a certain level. The reason is that the majority of uplink user channels, at least in WCDMA, are subject to power control. This power control aims at keeping the received power level of each channel at a certain signal to interference ratio (SIR), in order to be able to meet specific service requirements. This SIR level is normally such that the received powers in the radio base station (RBS) are several dB's below the interference level. De-spreading in so called RAKE-receivers then enhance each channel to a signal level where the transmitted bits can be further processed, e.g. by channel decoders and speech codecs that are located later in the signal processing chain.
Since the RBS tries to keep each channel at its specific preferred SIR value, it may happen that an additional user, or bursty data traffic of an existing user, raises the interference level, thereby momentarily reducing the SIR for the other users. The response of the RBS is to command a power increase to all other users, something that increases the interference even more. Normally this process remains stable below a certain load level. However, in case a high capacity channel would suddenly appear, the raise in the interference could lead to an instability, a so called power rush. This explains why it is a necessity to schedule high capacity uplink channels, like the enhanced uplink channel in WCDMA, so that instability is avoided. In order to do so, the momentary load must be estimated in the RBS or any node connected thereto. This enables the assessment of the capacity margin that is left to the instability point.
The load of a cell in e.g. a CDMA system is usually referred to some quantity related to power, typically noise rise or the rise over thermal (RoT). Power quantities, such as total power level and noise floor (ideally thermal noise), have to be determined. Determinations of highly fluctuating power quantities or noise floor according to prior art is typically associated with relatively large uncertainties, which even may be in the same order of magnitude as the entire available capacity margin. It will thus be very difficult indeed to implement enhanced uplink channel functionality without improving the load estimation connected thereto.
A number of noise rise measures do exist. The most important one is perhaps the Rise over Thermal (RoT) that is defined as the quotient of the total interference of the cell and the thermal noise power floor of the receiver of the RBS. Other measures include e.g. in-band non-WCDMA interference with respect to the thermal noise floor.
At this point it could be mentioned that an equally important parameter that requires load estimation for its control, is the coverage of the cell. The coverage is normally related to a specific service that needs to operate at a specific SIR to function normally. The uplink cell boundary is then defined by a terminal that operates at maximum output power. The maximum received channel power in the RBS is defined by the maximum power of the terminal and the pathloss to the digital receiver. Since the pathloss is a direct function of the distance between the terminal and the RBS, a maximum distance from the RBS results. This distance, taken in all directions from the RBS, defines the coverage.
It now follows that any increase of the interference level results in a reduced SIR that cannot be compensated for by an increased terminal power. As a consequence, the pathloss needs to be reduced to maintain the service. This means that the terminal needs to move closer to the RBS, i.e. the coverage of the cell is reduced.
From the above discussion it is clear that in order to maintain the cell coverage that the operator has planned for, it is necessary to keep the interference below a specific level. This means that load estimation is important also for coverage. In particular load estimation is important from a coverage point of view in the fast scheduling of enhanced uplink traffic in the RBS. Furthermore, the admission control and congestion control functionality in the radio network controller (RNC) that controls a number of RBS's also benefits from accurate information on the momentary noise rise of the cell.
One approach to improve load estimation is disclosed in the published international patent application WO 2006/076969. A minimum value of a power quantity, preferably a difference between the instantaneous total received wideband power and the instantaneous sum of powers of all links used in the same cell, is used as an estimate of an upper limit of the thermal noise floor. An optimal and soft algorithm for noise rise estimation based on a similar basic idea of minimum values is disclosed in the published international patent application WO 2007/024166. Complexity reduction procedures concerning such algorithms are further disclosed in the published international patent application WO 2007/055626.
Admission control makes sure that the number of users in a cell does not become larger than what can be handled, in terms of hardware resources and in terms of load. A too high load first manifests itself in too poor quality of service, a fact that is handled by the outer power control loop by an increase of the SIR target. In principle this feedback loop may also introduce power rushes, as described in the previous section.
The admission control function can prevent both the above effects by regulation of the number of users and corresponding types of traffic that is allowed for each cell controlled by the RNC.
In order to regulate the number of users the RNC needs to have means for computation of a measure of the load of a cell. This measure of the load of the cell is then compared to a threshold, and new users are accepted if the load of the cell is predicted to remain below the threshold, after the tentative addition of the new user. An improved load measure for the admission control function is requested, so that a higher number of users can be accepted, without sacrificing cell stability limits.
One approach for increasing the useful capacity is to utilize different kinds of receiver diversity. By using more than one receiver branch, radio signals that are not entirely correlated can be achieved. MIMO (multiple-input-multiple-output) and diversity combining algorithms combine the signals from several receiver branches, in order to enhance the overall performance of the receiver. Some, but not all, combining methods implicitly assume that the receiver branches are power balanced, i.e. calibrated. The problem of calibration of time is at least equally important and has received a substantial amount of attention. That problem is however beyond the scope of the present invention disclosure.
The relevance of a correct power balance, i.e. a correct power calibration, is evident when considering fusion of two received signals. Two signals with the same signal-to-noise ratio, where one of the signals has a significantly reduced amplitude as compared to the second signal, caused e.g. by an un-calibrated receiver chain, are not easily combined. In case the receiver scale factor errors are not corrected for, the combined signal will evidently suffer from degradation.
To handle the above calibration problem, channel estimation can be applied individually for each receiver branch. In that way, any unknown scale factor errors of the receiver chain are incorporated into the channel model of each receiver branch. Note that this approach would require the use of a training sequence, and a successful decoding in case decision feedback is applied for channels estimation, e.g. jointly with turbo decoding. Such approaches are, however, complex.